Taking a closer look at the optical fibre that’s used to build our communications network.

Welcome to Building Fibre where we take an inquisitive look at how creating a smart, connected world, is impacting the way we design city infrastructure.

When starting a podcast series that’s fundamentally based around optical fibres, I guess the best place to start is to ask the question “what is an optical fibre?” and why is it so important. An optical fibre can sometimes made of plastic but most often it’s made of glass. But when you explain to people that a cable is made from glass you occasionally get a curious or disbelieving response.

Glass is a remarkable creation, made all the more amazing by a common belief that glass is a tough but fragile material that can be broken with a good, sharp blow. Every-day glass, like a window pane or a bottle is stiff and brittle but when you manufacture glass at a micro scale, where it’s melted and then drawn out into a long thin fibre that’s around the thickness of human hair, it behaves differently, it becomes flexible.

Glass is made of silica and without getting too technical silica is the main constituent of sand. So there’s a fair amount of raw material around and it’s reasonably easy to extract.

Archaeological evidence suggests glass-making dates back to at least 3,600 BC around Mesopotamia, Egypt and Syria with the earliest known objects being glass beads perhaps created accidently during an early metal working process.

Over the years glass has been refined and transformed into an amazing array of functional products and has a great many practical, technological and decorative uses.

From common glass used in the afore mentioned windows and table ware, along with packaging, jewellery, decorative ornaments and sculptures through to technically refined glass where its spectacular properties are employed to create everything from the cladding on immense buildings like sky scrapers or the Shard, down to fine micro fibre glass strands used to make glass wool for building insulation.

But our interest today focuses on the use of glass within the field of communications and the very special part it plays when light passes through it.

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Glass, as a medium, has the property of allowing light to transmit or pass through its structure. But light travels at different speeds depending upon the medium it’s travelling in. Light travels fastest in a vacuum, such as the light that comes from the Sun, which travels at over 182,000 mile per second. But when it hits the Earths atmosphere it changes speed as it passes through the air, a different medium. The speed changes again as the light passes from air to glass and from glass to water and so on.

When light passes from air to glass the speed at which it travels is reduced and if it enters at an angle the direction of the light rays is bent due to an effect called refraction. Each material is said to have a refractive index, a number which tells how the light will react and not only is it different for each material, but it can be controlled by managing the composition of the material, and in our case that’s the glass. If the angle of entry reaches another critical point then the light is not just bent but it’s reflected and we put both of these effects of refraction and reflection to very good use.

Light doesn’t pass through any medium completely unimpeded. Impurities or particles can scatter the light, reducing the amount that gets through and light itself, being an electromagnetic wave, has its own wonderful properties that impacts its progress and occasionally create effects that can defy belief. But more about that in another episode.

You can easily show the effects of  refraction and reflection with a couple of simple experiments.